In a usual image-forming method using an electrophotographic photoreceptor according to Carlson's method, an image is formed by the procedure comprising the steps of uniformly charging the surface of a photoreceptor, imagewise exposing the surface to form an electrostatic latent image, developing the latent image by a developer containing a toner to form a toner image, transferring the toner image to an image receiving sheet, and fixing the toner image on the sheet.
On the other hand, the photoreceptor is subjected to cleaning process for removing toner remained on the surface, and is discharged for standing by the next image forming operation. Thus the photoreceptor is repeatedly used for a prolonged term.
Accordingly, the photoreceptor is required to be excellent in physical properties such as copying durability, resistivity to abrasion and resistivity to humidity, and in resistivity to ozone generated at the time of corona discharge and to UV ray at the time of light exposure as well as in electrophotographic properties such as electrification property, light-sensitivity and dark decay.
An inorganic photoreceptor using an inorganic photoconductive substance such as amorphous silicon or selenium as a main composition, is widely used. Recently, however, an organic photoreceptor using an organic photoconductive substance becomes to be used, which is low in cost, and excellent in workability, and has a wide selection degree of freedom according to use.
Although various cleaning methods has been known for cleaning the toner remaining on a photoreceptor, such as a magnetic brush method and a fur brush method, a rubber elastic cleaning blade is principally used since which is simple in the constitution and is excellent in the cleaning effect. As the form to contact the rubber elastic cleaning blade to the photoreceptor, trailing method and counter method has been known. Among them, the counter method is mainly used in the reason of that this method is superior in the cleaning effect. In the counter method., the blade is contacted to a photoreceptor so as to make an acute angle against to the moving direction of the photoreceptor as shown in FIG. 2.
For developing an electrostatic latent image formed on the photoreceptor, a single-component developer mainly composed of magnetic toner particles having a size of 1 to 30 .mu.m, or a two-component developer composed of non-magnetic toner particles having a size similar to that of the magnetic toner particles and magnetic carrier particles having a size of 10 to 100 .mu.m are usually used. The toner particle comprises a binder resin, a colorant dispersed in the resin, and a releasing agent such as a low-molecular weight wax, the particle further contains a magnetic powder dispersed in the resin if it is necessary.
On the other hand, with respect to the organic photoreceptor, there are problems, since the surface of the organic photoreceptor is softer than that of an inorganic photoreceptor and the fine toner particles or an addendum thereof is tend to be tightly adhered on the surface of the photoreceptor. The adhered substances are difficult to remove. Further the surface of the organic photoreceptor is tend to be worn away and the electrophotographic properties of the photoreceptor is degraded when the photoreceptor is subjected to cleaning operation, with a strong pressure the same as for the inorganic photoreceptor.
Japanese Patent Publication Open for Public Inspection (JP O.P.I.) No. 6-130711/1994 (Publication 1) proposes a countermeasure to the above problems, in which a photoreceptor having a protective layer on an organic photoconductive layer is used. The protective layer contains 5.0 to 70.0% by weight of fine particles of a fluororesin, and has a surface roughness of 0.1 to 5.0 .mu.m, a surface hardness by Teber method of 0.1 to 20.0, and a surface friction coefficient of 0.001 to 1.2. The photorecepter is subjected to cleaning by a rubber-elastic blade which is contacted to the photoreceptor with a line pressure of 20.0 to 50.0 g/cm. The publication describes that the wearing off and the defect formation on the surface of the photoreceptor, and insufficient cleaning caused by turning over the cleaning blade are prevented by the above technique. As result of that, the durability of the photoreceptor can be raised, and an excellent image can be obtained.
Further, for example, JP O.P.I. Nos. 3-264961/1991 (Publication 2) and 2-296067/1991 (Publication 3) describe that the releasing agent contained in the toner of the developer strongly relates to the cleaning condition of the photoreceptor. The above Publications 2 and 3 each describes that the formation of finely crushed toner and filming of toner can be prevented when the domain size of the releasing agent is not more than 5000 .ANG. and 2000 .ANG. to 3000 .ANG., respectively.
A comprehensive study on the properties of surface of photoreceptor, cleaning member and toner for developer are indispensable to solve the problems of cleaning of remaining toner on the photoreceptor surface. It is considered that the sufficient cleaning would be attained after these properties have been made clear. However, in the disclosure of the above Publication 1, technical attention is devoted only to the roughness, hardness and friction coefficient of the photoreceptor surface, and property of the cleaning blade and that of the toner are not described at all.
As above-mentioned, the techniques of Publications 2 and 3 are attained by paying attention to the releasing agent in the characteristics of the toner. In the publications, it is described that formation of finely crushed toner can be prevented when the maximum domain size of releasing agent existing in the resin in an insular form, is not more than 5000 .ANG., particularly within the range of 2000 to 3000 .ANG.. The finely powdered toner is tightly adhered to the surface of photoreceptor and the adhered toner is difficult to removed. As a result, the filming of on the photoreceptor caused by fine powdered toner and degradation in the electrophotographic property of the photoreceptor by fatigue can be prevented.
However, it has been found by the inventors, that the cause of filming of a foreign substance on the photoreceptor is changed depending on the kind of photoreceptor and composition of the toner to be used. Particularly, in the process using the organic photoreceptor, the filming is mainly caused by adhering the releasing agent broken away from the toner rather than adhesion of the finely crushed toner.
Namely, the releasing agent domain contained in the toner is broken away from the toner and adheres on the photoreceptor surface in a form of film in the course of image forming by the organic photoreceptor. The film formed on the photoreceptor degrades the cleaning effect of the cleaning blade and causes adhering powder of metal or paper, addition to the toner, on the photoreceptor surface. The adhesion of such foreign substances causes degradation in the electrophotographic property of the photoreceptor and formation of image defects such as white spots, black spots and black streaks.